With the notable exceptions of mineralocorticoid antagonists and epithelial sodium channel (ENaC) inhibitors, most clinically useful diuretics/natriuretics (for example, thiazide-type, thiazide-like, and loop diuretics) increase potassium excretion, leading to increased risk of hypokalemia (Reilly R F et al., Goodman & Gilman's the pharmacological basis of therapeutics, 671-719, 2011). In addition, the tendency of thiazide-like/type diuretics to increase plasma glucose has been widely discussed and remains a concern (Palmer B F et al., Seminars in Nephrology, 31:542-552, 2011; Gilbertsen R B et al., Annals of the New York Academy of Sciences, 451:313-314, 1985). As diabetics frequently also suffer from hypertension, such patients would benefit from diuretics that do not increase glucose levels (Sowers J R et al., Hypertension, 37:1053-1059, 2001). Thus, a need exists for a diuretic/natriuretic that does not lead to increased potassium excretion and/or an increase in plasma glucose.
Peroxynitrite (ONOO−) is formed in vivo from the diffusion-controlled reaction between superoxide anion (O2.−) and nitric oxide (NO) (Carballal S et al., Biochimica et Biophysica Acta, 1840:768-780, 2014). Further, ONOO− is a highly reactive nitrogen species (RNS) that can mediate nitration (i.e., insertion of —NO2) of a number of endogenous compounds, including those containing a guanine moiety (Ohshima H et al., Antioxidants & Redox Signaling, 8:1033-1045, 2006; Szabo C et al., Nitric Oxide, 1:373-385, 1997; Yermilov V et al., FEBS Lett, 376:207-210, 1995). In this regard, ONOO− nitrates guanine moieties at position 8 of the purine ring to produce 8-nitroguanine units in DNA, RNA, and the guanine nucleotide pool (Ohshima H et al., Antioxidants & Redox Signaling, 8:1033-1045, 2006; Szabo C et al., Nitric Oxide, 1:373-385, 1997; Yermilov V et al., FEBS Lett, 376:207-210, 1995). It is also conceivable that free guanine per se could be subjected to nitration at the 8 position. In addition to RNS-mediated modification of guanine-containing compounds, reactive oxygen species (ROS), such as O2.−, can also modify position 8 of guanine moieties by inserting a hydroxyl functional group (Szabo C et al., Nitric Oxide, 1:373-385, 1997; Misiaszek R et al., Journal of Biological Chemistry, 279:32106-32115, 2004).
After modification of guanine moieties by RNS or ROS, subsequent catabolism of RNA, DNA, and the guanine nucleotide pool will release 8-nitroguanosine, 8-nitro-2-deoxyguanosine, 8-hydroxyguanosine, and 8-hydroxy-2-deoxyguanosine. Theoretically, reduction of 8-nitro groups could yield 8-aminoguanosine and 8-amino-2-deoxyguanosine, and purine nucleoside phosphorylase (PNPase) can convert such compounds into 8-aminoguanine (Osborne W R et al., Immunology, 59:63-67, 1986). In addition, PNPase might convert 8-nitroguanosine and 8-nitro-2-deoxyguanosine into 8-nitroguanine, and reduction of 8-nitroguanine would yield 8-aminoguanine. Similarly, PNPase might produce 8-hydroxyguanine from 8-hydroxyguanosine or 8-hydroxy-2-deoxyguanosine. Taken together, these considerations suggest the metabolic framework summarized in FIG. 1. Consistent with this framework are studies confirming the presence of 8-nitroguanosine, 8-aminoguanosine, 8-aminoguanine, 8-hydroxyguanosine, 8-nitroguanine, 8-hydroxyguanine, and 8-hydroxy-2-deoxyguanosine in tissues or urine (Akaike T et al., Proc Natl Acad Sci USA, 100:685-690, 2003; Sodum R S et at, Chem Res Toxicol, 6:269-276, 1993; Park E M et al., Proc Natl Acad Sci USA, 89:3375-3379, 1992; Ohshima H et at, Antioxid Redox Signal, 8:1033-1045, 2006; Fraga C G et al., Proc Natl. Acad Sci USA, 87:4533-4537, 1990; Lam P M et al., Free Radic Biol Med, 52:2057-2063, 2012).